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Magazine reader Robert is planning to build ten different mods of the Compressed Air Rocket (suggested by the Cub Scouts in his troop) and test them at an upcoming Cub Scout picnic, and he’s looking for ideas on how to gauge performance between the different versions.

I took the new launcher to our Cub Scout den meeting and it quickly turned in to an opportunity to get most of the kids an engineering requirement for one of the badges everyone is working on. The kids all sat around throwing out ideas to improve performance and or durability. I was really surprised by some of the things they came up with. So, I agreed to build their 10 favorite mods in time to test them at the Scout picnic on the 23rd. So far the only idea I’ve come up with to gauge performance between rockets is to time the interval between launch and impact. I was wondering if you might have any other ideas?
Thanks in advance, Robert.
P.S. I refused to make the most popular idea no matter how many times they asked. The idea was to place a very large nail in the nose and you would be amazed at how many “practical” reasons a group of 10 year old’s can come up with! I grew up in the age of “Lawn darts”, they banned those things for a reason!!

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I have really enjoyed this build. Two things that I changed was wiring two 9V in series to get a bit more voltage and it means I don’t have to charge the RC batteries or mess with different types of connectors. The other thing I saw posted on one of the forums was to use foam pipe insulation and then wrap it in colored Duct tape. This proved to less time consuming and the rockets could stand many many more launches until it was finally lost on somebody’s roof. I found some of my younger children less coordinated and making the paper rocket was a bit much for them. Trajectory was still pretty amazing with the extra bit of weight of foam yet still maintaining some element of safety.

John

Well I assume you’ll be trying to maintain the launch pressure consistent across all launches. The method I’ve found that work for me is to use an external portable air tank with independent regulator. I charge up the tank to 125psi at home. I have a regulator on the tank and I dial that down to usually 30psi (we launch in a very small city park in DC so I want to make sure the rockets stay close, but you could dial it up if you wanted, but the higher the pressure, the less launches you get from the tank), and then I can be sure that all charges of the launcher come out the same. I do this for fast launches when out with the kids, and it keeps it mindless and consistent. It helps when there’s a large group of kids since I just push the air hose to the valve on the launcher and wait for the pressures to equalize. I have a gauge on the launcher as well so I can make sure it matches up with the gauge on the regulator.

If you wanted an easy way to keep the launch timer equalized, I’d rig up the launch switch to trigger the start/stop of the timer as well if you could. you could probably make an easy timer setup with an lcd, a few resistors and an arduino tied to pickup the high voltage pulse of the launch switch. it’d be more consistent for triggering the start of the timer synchronized with the rocket launch. then the only variable would be the end switch when you see it hit the ground.

JWC

I made a mod on the launcher by attaching it to a small air compressor. The compressor maxes out at 100 psi, which is well below the pipe’s rating, so safety isn’t an issue any more than with the hand pump. The other nice things about this setup is (1) We can rapid-fire rockets without anyone getting tired and (2) if the air compressor isn’t attached the launcher won’t hold any pressure – which makes it a nice, visual safety.

Rick Schertle

JWC,

Good ideas. I use a small rechargeable compressor for local park launches and my big compressor for large events like Maker Faire. With the large compressor, it automatically recharges after every launch, which is good because we often do 2000-3000 a day!

– Rick

Ellis in West Valley, UT

I enjoyed reading this! Being a model rocketeer and scouter, I’ve flown solid propellant, and compresses air/ water powered rockets, but not a strictly air powered rocket, except for the toy made by whamo with a foam rocket. but that was not for performance. the best way to gauge performance is height, which correlates to time from launch to landing. you may want to try different nose cone shapes (cone, rounded, ogive, etc) and different fin configurations (shapes and number of fins). perhaps length of body tube might show some different performance because of a lighter body mass with a shorter tube. love to hear how it turns out!

Your idea to measure time to impact is a good one with pretty obvious problems. Most of these can be avoided with a weighted streamer. Cut a long strip of plastic (from a garbage bag should work) and attach a small weight. Since there is lots of drag from the streamer it will reach its low terminal velocity quickly after being released from the rocket. Simply add a release mechanism similar to how a parachute works and record the amount of time between release and impact.

The other option is to use optical tracking. Take a large 180 degree protractor attach a string with a weight to the center point. Then have the observer stand a fixed distance away from the launch pad. As the rocket flies up the observer keeps the straight edge of the projector on a straight line between his eye and the rocket, once it reaches its maximum height record what angle the weighted string is at. Use some trigonometry to figure out the altitude the rocket reaches. (you know two angles and one length, don’t forget to take the height of the observer into account).

Good luck!

http://brianandandie.wordpress.com brianandandie

I have played around with various rocket fin designs, and I keep one rocket that was built entirely out of duct tape in the box that is my compressed air rocket kit, for “motivation/demonstration” I also upgraded to 2 9V batteries in an altoids tin that’s strapped to the base. I also changed the pressure chamber design slightly so that it looks more like a “T” than an “L”, and I added more simple adjustable legs made out of 2×4 and attached to the pressure chamber using U bolts, so I can still adjust the trajectory and have to stay in place. There are a handful of companies out there that make “ice cube lights” which are essentially waterproof “throwies” or blinking LEDs that we’ve taped to the sides of the rockets in pairs. They have to be in pairs to keep from affecting the flight path, and then you can extend you launching time past dusk a ilttle :D

Happy Rocket Launching!

Jon Howard

I replaced solid fuel rockets with compressed air this year in a middle school technology class I teach and never looked back. Couple things I figured out:

1. My favorite design uses gummed packaging tape (the kind you wet to stick) wrapped around a piece of paper (and the pvc lauch tube as a form). The tape is wrapped like you would wrap a tennis racket handle and then the end is sealed with hot glue when the tube has dried. The end sealing was a key to success. Press in opposite sides of the tube so they just cover each other leaving the other sides up like batman ears. Squirt in some hot glue on the bent sides and then fold the remaining two sides (the batman ears) over on top of the hot glue using the pvc tube to flatten it. The result is a hot glue sealed end that gives a nice front load and is resistant to crashes. Then we formed disposable nose cones with the same gummed tape and attached them by simple wetting the tape. The nose cones lasted only a launch or two, but were easy to make. I never had a rocket explode using this design. This design allows for easy mods to test weight (using extra layers of tape), body length, nose cone designs, fin designs etc without changing any other variables. We also had an open division where they could use any material. But my design ended up being the best and the others exploded regularly.

2. We started by testing using time in the air, but I felt it was too inexact. The light ones seemed to float a bit and with a student running a timer, human error is a factor. We ended up doing tests on a football field launching at 45 degrees and going for distance using a set launch pressure (we used a pump with a gauge, but a compressor would be better). It worked great and I felt like it was a much more accurate gauge for measuring changes in design. My design ended up going end zone to end zone at 80 psi. They also did field goals which was fun.

Rick Schertle

Jon,

We do these a the middle school where I teach as well! Thanks for posting your ideas.

– Rick

k-twizel

Performance issues with my set-up were of interest as well but for different reasons… my kids are young and don’t have the attention span for a lot of ‘troubleshooting’. I looked at it from the stand point of “What is the best design of a rocket to fly the highest and takes the least amount of time to build?”

[My brilliant plan was to do an Instructable on my rig but the weather is not going to cooperate with me this weekend.]

My rocket (if all supplies are available) takes about 10 min to build, will launch aprox, 100′ @ 75-80 PSI, and is very reusable (20-30 flights per rocket… haven’t reached end-of-life for the first set)

I will save the details for the Instructable but the rocket is 2 toilet paper rolls taped with clear packing tape, a foam practice golf ball (cut in half), and kid-personalized drawing wrapped around the body.

You could (carefully) launch horizontally into a target and measure penetration. Preferably this would be done towards the side of a hill or the edge of a hole (like towards the wall from the bottom of a quarry)

It’s pretty much the same thing as you would look for for a safe rifle practice location.

http://gravatar.com/nerdama nerdama

This is a bit off of your post request but…Some Mods we’ve played with are:

-Helicopter (whirly) recovery with ‘blades’ that deploy on descent. make them of stiff cover stock, use string to keep them at about right angles when deployed. Works sometimes… but is really cool when it all comes together. Be sure to weight the bottom of the rocket.

-Nose cone glider, make a stubby paper airplane glider to sit on the nose cone that is secure on ascent but can pop off and glide down on descent.

-Replace the paper/masking tape with card stock and duct tape. Go for durability on how many launches you can get out of it at 40psi until it just falls apart.

-Bomber: Put a ping pong ball in a cup on the tip of the nose cone, go for low-PSI launch at a bit of an angle. See if you can get close to a point in the field with the ping pong ball when it falls out. Random fun. Very low altitude usually.

-Something I haven’t tried, but would like to: Get an Arduino Mini and an accelerometer and do some data logging on how many g’s you get… maybe a cheap camera. Stuff like that. if you do this, hook up button cells to keep the weight down. Don’t do the Arduino but hook up a series of LEDs along the side of the rocket, maybe a flasher circuit. Take apart an FM Microphone transmitter (toy style) mount it into the rocket nose cone, bring a boom box to tune to the transmitter frequency and ‘hear’ your rocket roar.. See how far your signal will broadcast.

-AirRocket Flugtag – Build a ridiculous rocket, painted, with big wings.. maybe a propeller, glue on action figures, Paint it with an awesome slogan, add lasers. The goal here is not efficiency but STYLE. Take pictures before launch, stand back far away. Rapid Spontaneous Dis-assembly is possible.

— As for tests for performance improvement and mods to try:
Different lubricant (dry is my recommendation) on the launch tube. Baby Powder,Silicone Spray etc..

Set the pressure at a standard and the launch angle at a fixed angle (10-15 degrees), measure distance. Assuming wind is not a big factor you can use airtime, distance traveled from launch and some math to calculate height.

Try moving Fin placement up and down on the tube. Fins right at the end of the tube don’t seem to work as well as a bit up on the body. I don’t know why.

I love our launcher.. this was a good reminder that summer is here and I need to bust it out again!

Rick Schertle

nerdama,

I want to hear more, especially on the heli-recovery. I’ve also been looking into simple ways to deploy a parachute. With a recovery system, all kinds of opportunities open up like launching small video camera. Glad you’re enjoyed the project so much!

Cheers,

Rick

Nardella

You could also go for distance instead of height.

Mike F

I would use a ratio of air pressure to altitude as an efficiency rating. Model rocket altimeters are cheap, or a protractor with a weighted string works too (the even make apps that reflect the angle the phone is at). With some simple trigonometry (great way to show how useful math can be) you can have some fun competition.

http://www.facebook.com/rick.jesmer Rick Jesmer

To NOT make a lawn dart put A PARACHUTE IN THE rocket they do make them if not make one I see so meany good ideas but fall on the last thing. I do have some ideas but i’d like to try them out first but i can’t get some of the parts there are other things iv tryed to get but they say they don’t have them or wont sell them to me like radio shack for one. Makeing the rocket last lounger like strips of plastic along the sides make the rocket lite up at night and or spins with the lites with the PARACHUTE and you can find them thats one.

John Kuszewski

I did something related with my Webelos den a few years ago for their Scientist activity pin. Instead of paper rockets and an air compressor, we used those nerf guns that fire suction cup darts. I built some plywood protractors and duct taped the nerf guns to them. Then the boys fired darts at controlled angles with repeatable force (thanks to the nerf guns’ mechanism) and measured the horizontal distance the dart travelled. Upon graphing distance vs angle, they discovered that ballistic trajectories are longest at 45 degrees. One of my finest hours as a Cub Scout leader.

justin

Bravo, as a physics teacher I really appreciate this activity.

Rick Schertle

Hey Everyone! Excellent ideas and mods coming in so far. I want to take some time to read and reply to each, but for now, wanted to say thanks so much and keep them coming!

Cheers,

Rick Schertle
Project Designer for MAKE

http://www.facebook.com/bart.patrzalek Bart Patrzalek

if your going for efficiency go with a parabolic nose cone (lowest drag) also make the rocket as “slippery as possible” to reduce drag as well, as to length of the body tube and fin size you could do some simple calculations in order to get the least drag but make sure that the center of pressure is below the center of gravity to make the rocket stable, you can easily do this by adding some weight to the nose cone. as for measuring performance i would go with an altimeter (couple bucks at a hobby store) and have the same pressure in the tank each time (best if you do it with a gauge rather than counting how many pumps it takes)

Chuck Brannan

I am new to Arduino (What’s that got to do with rockets Chuck?), but I am reading up on wireless relays so that you can be rid of the cable running from your launch button to the sprinkler valve. That thing gets tripped over and yanked out all the time! I am exploring some micro controllers that can make the launch trigger wireless.

Thoughts?

Rick Schertle

Chuck,

I would like to hear more as you look into a wireless launch system. Right now, as a safety precaution, I hold the launch button while putting the rocket on the launch tube. I’ve had the rocket launched in my hand a couple of times so now I always hold the button. Thanks for posting your thoughts.

I’m with Hackerspace Charlotte and ever since I first saw this in Make we’ve been building launchers and rockets.

The best rockets we have use regular paper. Enlarge a 1/2″ piece of pipe with few extra layers of tape. Then roll a whole sheet a paper (no need to cut anything) around the tube and tape. The tape makes all the difference. We use clear packing take. It’s light, it’s slick, and it’s strong. As an added bonus if the kids want to decorate their rockets they can do that first and then you can coat with tape. We’ve launched these rockets at unsafe pressure (150+ lbs ) just to test the limits. They don’t fail. The fins are made of only packing tape (or occasionally a business card). We just fold about an inch of the tape on itself and stick the rest to the rocket. Then we cut the leading edge of the fin (lest you get erratic flight when the fins bend over at the leading edge). We put three of those on in less than a minute. We often skip the nose cone altogether – just cap the tube with 4 layers of tape (no paper disc).

That’s our basic rocket that we make in easily less than 5 minutes that will last until lost. But we have some popular mods:
— LEDs taped to outside
— stuff an LED inside towards the nose cone… since it’s clear tape
— glow sticks on the outside
— electrical tape for nose weight
— one angled fin for spin
— a streamer stuffed inside for a delayed deployment (we’re working on getting that as a drone for parachute)

I recently went the other way with testing… I took my kids out to test the effects of changing the volume of the compressed air storage chamber at a variety of pressures with rockets of differing weights. We even wrote done our results so we can call it science! The short answer is, for manual control (human at the switch) a relatively small volume (15 inches of 1″ tube) is sufficient for decent range ( 200-300 feet at 20 degree launch angle). We’re (the Hackerspace) working on an arduino controlled fire control system to for this years Fourth of July Fabulous Fireless Fireworks Display. Essentially we suspect we don’t need the valve to stay open very long to launch a rocket, so we can close it before we lose a lot of pressure. That will allow for more launches (those compressors have a hard time keeping up).

And speaking of fireless fireworks… We swap the 1/2″ pvc launch tube for a 1.25″ tube and load in glow sticks/bracelets (the 15 for $1 kind a Michael’s or Target). Wadding (paper towel) is required to keep the air from rushing around the sticks, and in between each layer. We typically have three layers of 25 sticks in each tube and launch at about 75lbs psi for 100′ vertical launches. If we wrap the 25 stick bundles in a paper towel they stay together longer and will go higher. Streamers on the glow sticks sort of work, but it’s not worth the effort. Those same glow sticks tape nicely to the outside of rockets too (remember, clear packing tape) and go insanely high – but they’re much easier to track at night. LED’s are even brighter – and very cool if you put different colors on one rocket and have it twist… or have some point up and some down – you see different colors on the way back down.

Our launchers have evolved quite a bit into very efficient, compact and cost effective models. I’ve build about 10 now with no two being the same, and some wildly different. I’ll see if I can’t get around to posing pictures and test data (and maybe some arduino code) up on the hackerspacecharlotte.org website before too long.

Rick Schertle

Quincy,

Thanks so much for responding to the post. I’m glad you’ve had such a good time with this project. It seems I’ve been spending so much time launching that I haven’t taken the time to work on mods. I’m especially interested in ideas for deploying a recovery system. I would LOVE to see some pics of some of the work you’ve done.

In September we’re doing a night launch with LED’s at the local children’s museum. Should be fun. I appreciated your testing at higher pressures. I’ve always believed this to be safe project if common sense is used. I think at the usual 75 psi, you are well below the tolerances of the PVC. I’m sure the valve would blow or tubing before the PVC went. Again, I would love to see some pics and thanks again for posting your mods.

Cheers,

Rick Schertle

http://slackalicious.wordpress.com slackalicious

Wow, comments from the man himself! Again, thank you so much for publishing your plans. I’ve had so much fun sharing these rockets with others. Last night I put up a quick block so I could share a couple pictures and not just have a dump of all the photos I’ve taken (though I tried that too).

It should be said that for our launchers we use screw in schrader valves which can handle considerably more pressure than tire valve stems. Valve stems are rated for 65psi or possible 110psi if they’re the “high pressure” (much more rare) variety. Home Depot has the valve stems online for only $1.49!! Shipping will kill you unless you’re ordering in bulk though. Grainger also has them for $3.04 each. Failing that Ace Hardware usually keeps a couple in stock, but they’re $5, and the brass busing to get from the 1/8″ threads on the schrader valve to the 1/2″ pvc threads costs another $3-$4 from just about any hardware store.

Rick Schertle

Quincey,

Thanks a ton for the kind words on the project I love your mods and your write-up on the blog was fantastic. We’re on a road trip across the US and I wish Charlotte was on our route so I could visit your local hacker space. All the best to you and all the folks there!

– Rick

http://cooperfreer.wordpress.com cooperfreer

wow very usefull information very great effort thanks and feel free to check myCompressed air